Current cellular wireless standards include stringent transmit power control requirements. For example, some orthogonal frequency division multiplexing (OFDM) standards (including variants of generic OFDM, such as Single Carrier Frequency Division Multiple Access (SC-FDMA) standards) include transmit power control requirements that deal with peak-to-average ratios (PARs) of 6.5 decibels (dB) or higher for both 16-QAM (Quadrature Amplitude Modulation) and Quadrature Phase Shift Key (QPSK) modulation. In order to address such requirements, a conventional wireless communication device may include a transmit power control system for amplification and transmission of a wireless signal. Such a system may include a power control feed-forward path and a transmit power detector feedback path. A power amplifier and variable gain amplifier (VGA) along the power control feed-forward path amplifies the signal to be transmitted. The transmit power detector feedback path measures the power of the amplified signal, and determines and provides control or bias voltages to apply to the system's power amplifier and VGA, thus controlling the transmit power ramp up and ramp down curves.
The emerging 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) standards specify that a single 3GPP LTE symbol is to be represented using QPSK, 16-QAM or 64-QAM, where 12 constellation points are selected as a data-dependent subset of a selected QAM constellation. In addition, under emerging 3GPP LTE standards, each transmit sub-frame may include a plurality of information-bearing symbols and two demodulation reference symbols located at fixed symbol positions within the sub-frame. In addition, occasional sub-frames also may include a sounding reference symbol (SRS) that replaces an information-bearing symbol of the sub-frame (e.g., the last symbol). Prior to transmission, the amplitude of the SRS may be transmitted at a different power level from the power level of the information-bearing and demodulation reference symbols.
Power measurement and control techniques for 3GPP LTE devices should be adapted to take into account this multi-level, modified QAM modulation. However, because a single 3GPP LTE symbol is represented using only a subset of available constellation points (e.g., 12 of 16 constellation points), not enough samples may be available to achieve sufficiently accurate power measurement and/or control using power measurement and control techniques currently employed in conjunction with conventional OFDM, 16-QAM modulation or other conventional power measurement and control techniques. In addition, conventional power measurement and control techniques are not adapted to take into account the multi-level modulation performed in conjunction with transmitting the SRSs.
Accordingly, methods and apparatus are desired for performing transmit power measurement and control for wireless communication devices in which insufficiently accurate power measurement and/or control may result from the use of conventional power measurement and control techniques. In particular, methods and apparatus for performing transmit power measurement and control are desired for wireless communication devices in which multi-level QAM modulation is performed (e.g., devices that employ a 3GPP LTE standard or another standard for which current power measurement and/or control techniques are unsuitable) and/or in which a data-dependent subset of available QAM constellation points are used to represent a symbol.